To recover data from noisy channels, such as for example magnetic data carriers having high data densities, it is known to class the write/read channel of the data carrier in accordance with a partial response characteristic which approximates to the frequency response characteristics of the channel, and the arrangement or design of a digital data recovery circuit is selected to optimise data recovery from a channel with that partial response characteristic. As the correct recovery of data is so dependent on how well the channel characteristics are matched by the form of data recovery circuit selected, it is common to provide an equaliser circuit on the input of the data recovery circuit to compensate for any difference between the actual and the approximated channel characteristics.
The data recovery circuit contains a phase-lock loop circuit arrangement which receives an analogue read signal, from the equaliser circuit if there is one, and operates to control an oscillator at the phase and an integer multiple frequency of components of interest of the incoming data stream. Signals generated by this oscillator are used to sample the incoming analogue signal at appropriate sampling points, from which samples data recovery is performed. Correct phase alignment of the oscillator signals and the components of interest of the analogue read signal are critical in performing correct data recovery.
To assist the phase lock loop circuit arrangement in achieving fast initial frequency and phase alignment, the data carrier will usually have one or more regions in which VFO field data has been intentionally written. The VFO field data is a regular data pattern which, when being read, provides an analogue signal which in approximately sinusoidal and periodic in nature. In using these data channels, it is known for the data to be encoded to contain a minimum of two consecutive like bits in the data stream, and the VFO field data may for example comprise a succession of pairs of like data bits.
It is widely appreciated that certain types of optical data carrier channels currently being investigated for development will have a response characteristic approximating that of equation (1). EQU F(D)=a+bD+bD.sup.2 +aD.sup.3 Eqn. (1)
Here, a and b are constant coefficients and D is a unit delay operator. This type of channel can be referred to as a class of partial response PR(a, b, b, a) channel.